1. Field of the Invention
This invention relates to a gas laser such as a discharge-excited gas laser and more particularly to apparatus for preventing an optical element, such as an output window and a mirror, of a gas laser from being contaminated by vapor.
2. Description of the Prior Art
A discharge-excited gas laser such as an excimer laser can oscillate in a pulse discharge mode at gas pressure of 2 or 3 atm, and produces an impulse wave during the discharge. The ions such as Al.sup.3+, C.sup.4+ generated from electrode materials have components which move in the direction of the optical axis. Therefore, these ions are diposited on the laser mirrors or the windows in the form of a thin film. The thin film suppresses the laser oscillation and causes a reduction in the power output, which in turn requires a frequent cleaning.
Hitherto, a contamination prevention method as shown in FIG. 2 has been proposed. More specifically, a gas generated in the laser tube 21 containing impurities is introduced into another container 23 where the impurities are trapped by an electrostatic cleaner 24. The purified gas is blown towards the mirror or the window, perpendicularly to the optical axis, so as to prevent deposition of the impurity on the mirror or the window 22. This method, however, requires a large-scaled apparatus and the cost is increased undesirably.
Thus, no means has been available which is inexpensive, easy to handle and effective in the prevention of deposition of the contaminant. This imposes a serious problem particularly in the case of a discharge-excited excimer laser.